staging: comedi: amplc_pci230: remove ai_chans member

All boards supported by the "amplc_pci230" driver have 16 AI channels,
so the `ai_chans` member of `struct pci230_board` is superfluous and can
be removed.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>